The present invention relates to the manufacture of integrated circuits. In particular, the present invention is directed to apparatus, systems, and methods for thoroughly mixing a plurality of gases to be used in a semiconductor wafer manufacturing process. More particularly, the present invention relates to thorough gas mixing for the formation of polycide films using materials such as tungsten silicide (WSi.sub.x) on a semiconductor wafer.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two-year/half-size rule (often called "Moore's Law") which means that the number of devices which will fit on a chip doubles every two years. Today's wafer fabrication plants are routinely producing 0.5 .mu.m and even 0.35 .mu.m feature size devices, and tomorrow's plants soon will be producing devices having even smaller feature sizes.
One of the primary steps in fabricating modern semiconductor devices involves the formation of a dielectric, metal, or insulating layers over a semiconductor substrate. As is well known, such layers can be deposited by chemical vapor deposition (CVD). CVD processes are particularly suitable for use with high integration devices because CVD layers provide superior step coverage and post-annealing qualities to those layers formed by sputtering or other conventional deposition methods. In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions (homogeneous or heterogeneous) take place to produce a desired film. In a plasma enhanced chemical vapor deposition (PECVD) process, the flowing gas may be excited to a plasma state. A controlled plasma is formed to decompose and/or energize reactive species to produce the desired film. The process of depositing layers on a semiconductor wafer (or substrate) usually involves heating the substrate and holding it a short distance from the source of a stream of deposition (or process) gas flowing towards the substrate. In general, reaction rates in thermal and plasma processes may be controlled by controlling one or more of the following: temperature, pressure, and reactant gas characteristics.
In the quest to achieve ever smaller devices, increasingly stringent process requirements are being imposed on integrated device manufacturing processes. One such requirement is the thorough mixture of process gases prior to introduction of the gases into a CVD chamber. A thorough mixture of the process gases is typically necessary to achieve a uniform deposition pattern on the semiconductor substrate. If the quality of the mixing achieved by the plurality of gases is insufficient, the CVD process using the gases will provide an uneven deposition pattern, which may result in variance of the sheet resistance of the deposited film, delamination during annealing, or other undesirable qualities which may degrade device performance.
A thorough gas mixture is particularly desirable in the formation of a tungsten silicide (WSi.sub.x) film. To overcome heat generation and heat dissipation problems associated with using conventional polysilicon films and scaled down device dimensions, "polycide" films using tungsten silicide have been formed by depositing a metal silicide layer over a layer of conventional polysilicon. Such polycide films have lower resistivities than polysilicon films and thus reduce heat generation and heat dissipation problems. The preferred method for forming tungsten silicide layers comprises chemical vapor deposition (CVD) of dichlorosilane (DCS) and tungsten hexafluoride (WF.sub.6) over a semiconductor substrate.
Unfortunately, despite the improved film characteristics of the DCS and WF.sub.6 process, the CVD process using these two gases is particularly sensitive to gas flow and mixture parameters. Conventional gas mixers adapted to provide adequate levels of gas mixing are costly to manufacture and sensitive to minor flaws associated with manufacturing. Conventional mixers typically only use one mixing step and rely on a mixer that is difficult to test prior to actual use in a semiconductor system. Hence, it would be desirable to provide an improved gas mixing apparatus that would provide reliable and thorough gas mixing. It would further be desirable to provide a gas mixing apparatus that facilitates manufacturing and would be less sensitive to minor flaws that may occur during manufacturing.